1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, in particular to a method of soldering an object to be soldered.
2. Description of the Related Art
In a method of manufacturing a semiconductor device called a power semiconductor module including semiconductor chips such as an insulated gate bipolar transistor (IGBT) and a free-wheeling diode (FWD), the soldering technique is often employed for joining the semiconductor chip and an insulated circuit board.
Most soldering techniques employ soldering flux in order to prevent the surfaces of the solder and semiconductor chips from oxidization and to improve wettability of the solder. The soldering flux, however, has a problem that the soldering flux contaminates an insulated circuit board and inside parts of a reflow furnace in which the solder is heated and melted. Accordingly, Patent Document 1 discloses a method for improving wettability of solder without using solder flux, in which the oxide film on the object to be soldered is removed with hydrogen gas introduced into the reflow furnace utilizing the reaction of reducing oxygen with the hydrogen gas: 2H2+O2→2H2O.
In the soldering technique disclosed in Patent Document 1, a sheet of solder is put on a joint surface of one of the two objects to be soldered, for example, an insulated circuit board, and the other object to be soldered, for example, a semiconductor chip, is put on the sheet of solder. The assembly is heated at a temperature higher than the melting point of the solder while supplying a reducing gas, hydrogen, for example, and then cooled down. In this soldering technique, gas of free radical, hydrogen radical, for example, supplied onto the solder and the object to be soldered reduces the oxide contained in the molten solder and at the same time reduces the joint surface of the objects to be soldered to clean the surface. This procedure improves wettability of the solder.
In order to enhance the reducing reaction with the hydrogen in this technique, the joint surfaces need to be effectively supplied with hydrogen gas in the process of melting the solder. If a flat solder plate is placed between an insulated circuit board and a semiconductor chip, the solder plate contacts without a gap with the insulated circuit board and with the semiconductor chip. Thus, hydrogen gas is not sufficiently supplied onto the joint surfaces of the insulated circuit board and the semiconductor chip.
Accordingly, Patent Document 1 discloses a soldering method that forms projections at outer peripheral places of the solder sheet to increase an area of the junction surface contacting hydrogen gas, thereby enhancing the reducing effect. As recent operating environment for power semiconductor modules became severe, the solder joint surface also became required for higher strength and higher heat resistance. Accordingly, solder material of a tin-high antimony (Sn—Sb) system is being applied in place of the traditionally used solder material of the tin-silver (Sn—Ag) system.
The solder material of tin-high antimony system, however, forms a thick oxide film on the surface of the solder plate and shows poor wettability. As a result, a problem is reported that traditional soldering techniques cannot sufficiently reduce the oxide film on the surface and a void is apt to be generated due to the oxide film on the solder joint surface. Under a condition of actual operation of a power semiconductor module connected to a load, in particular, if a void is generated on the solder joint surface in the central region of the semiconductor chip, the temperature of the heated semiconductor chip rises due to the thermal resistance caused by the void resulting in deterioration of performance of the power semiconductor module.
In order to reduce the void on the solder joint surface, Patent Document 2 discloses use of a solder material formed to a frame shape. In the method, a frame-shaped solder material is disposed between the joint surface of a first joining member and the joint surface of a second joining member and then fused to confine the atmospheric gas within the space at the joining region formed by the first joining member, the second joining member, and the molten solder material. The gas confined in a void in this additional step blows outwardly with a great force in the subsequent decompression step, resulting in reduction of the void included in the solder.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2009-272554
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2006-116564
The inventors of the present invention have made extensive studies about application of the conventional soldering methods mentioned above to the use of the solder material of the tin-high antimony system and have found the following problems. When the soldering method disclosed in Patent Document 1 was first applied to the solder material of tin-high antimony system, which forms a thicker oxide film on the surface than traditional solder materials, the wide surface area of the solder sheet having protrusions thereon increased the amount of oxide film in the solder and made sufficient reducing effect impossible only by increasing the area contacting hydrogen gas. As a result, a void was generated due to the oxide film remained on the joint surface between the semiconductor chip and the solder, deteriorating the joint condition.
When the soldering method disclosed in Patent Document 2 was applied to the solder material of tin-high antimony system, which exhibits higher viscosity than the traditional solder materials, the step to confine the atmospheric gas within the solder rather held the gas in the solder, generating a void.